Microlithography masks including image reversal assist features, microlithography systems including such masks, and methods of forming such masks

ABSTRACT

Microlithography masks are disclosed, such as those that include one or more image reversal assist features disposed between at least two primary mask features. The one or more image reversal assist features may be defined by a patterned relatively non-transparent material on a mask substrate. Microlithography systems include such masks. Methods of forming microlithography masks are also disclosed, such as those that include patterning a relatively non-transparent material on a mask substrate to form at least one image reversal assist feature located between at least two primary features.

TECHNICAL FIELD

Embodiments of the present invention generally relate tomicrolithography masks and systems, and to methods of forming suchmasks. More particularly, embodiments of the present invention generallyrelate to the use of assist features in microlithography masks andsystems for improving the definition of primary features formed usingsuch microlithography masks and systems.

BACKGROUND

The incorporation of increasing numbers of discrete devices (e.g.,transistors, conductive lines, conductive contact pads, etc.) intoprogressively smaller integrated circuits remains an important challengein the manufacture of semiconductor devices, such as memory devices andelectronic signal processors.

Many such discrete devices are fabricated using microlithography.Briefly, and in general terms, in photolithographic processes, aphotolithographic mask, which is often referred to in the art as a“mask,” is formed that includes a desired pattern corresponding to aparticular pattern that is to be transferred (e.g., “printed”) to alayer of material on a semiconductor die or wafer. The pattern generallyincludes optically transparent areas and optically opaque areas that aresuitably arranged on an optically transparent supporting substrate. Themask may then be interposed between an illumination system and a layerof an illumination-sensitive photoresist material applied to asemiconductor wafer. The illumination system emits illuminationradiation through the mask and onto the photoresist material. The maskallows certain regions of the photoresist material to be exposed to theillumination radiation while shielding other regions of the photoresistmaterial from the illumination radiation, in accordance with the patternof the mask. The exposure of certain regions of photoresist material tothe illumination radiation results in changes to the properties of thephotoresist material in those exposed regions. The photoresist materialis then “developed,” which results in removal of either the regionsexposed to the illumination radiation or the regions that were shieldedfrom the illumination radiation. As a result, the photoresist materialis provided with a pattern corresponding to that of the mask. Thesemiconductor die or wafer, with the patterned photoresist material thenmay be further processed in any number of ways to further fabricatediscrete devices on or in the die or wafer.

The illumination radiation may be monochromatic. When a wavelength ofthe illumination radiation is greater than a minimum feature size of apattern to be transferred to a photoresist material using a mask,various optical effects may adversely affect the quality of theresulting features formed on or in the die or wafer using the patternedphotoresist material. For example, edges between transparent areas andopaque areas on a mask may contribute to diffraction of the illuminationradiation, which may result in interference of the waves of illuminationradiation after passing through the mask, resulting in exposurereduction in areas intended to be exposed, and exposure in areasintended to be shielded from exposure. As feature sizes in semiconductorstructures decrease, diffractive effects, as well as other opticaleffects become more prominent limiting factors in microlithography.

Accordingly, various compensation methods are available that mayincrease the pattern fidelity in the structure. For example, in oneknown method, optical proximity correction (OPC) may be used to perturbthe shapes of transmitting apertures, or other features on the mask toenhance optical resolution in the sub-wavelength regime. In general, theperturbed features on the mask are sub-resolution features since theyare generally not printed onto the structure during the exposureprocess. Accordingly, such features are often referred to assub-resolution assist features. Examples of sub-resolution assistfeatures include “serifs” for reducing corner rounding in featuresformed in the structure, and “hammerheads” for reducing the shorteningof end line features. Other sub-resolution assist features includescattering bars, or “outriggers,” and “inriggers” that improve linewidth control in the structure. Still other methods may be used toimprove the resolution of features in the sub-resolution regime. Forexample, Phase Shift Masking (PSM) methods generally enable transparentareas on the mask to transmit phase-shifted illumination to thestructure in order to reduce destructive interference that may occurbetween transparent areas that are separated by an opaque area on themask. Still other methods may be directed to the illumination systemitself. For example, an incident radiation angle (σ) and/or thenumerical aperture (NA) of a projection lens may be suitably configuredto resolve relatively dense lines and spaces.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an embodiment of amicrolithography system of the invention.

FIG. 2A is a simplified plan-view of a portion of an embodiment of amicrolithography mask of the invention that includes image reversalassist features.

FIG. 2B is a cross-sectional view of the microlithography mask of FIG.2A taken along section line 2B-2B shown therein.

FIG. 3A is an enlarged view of a portion of FIG. 2B.

FIG. 3B is a graph illustrating an intensity of electromagneticradiation that may be present on the semiconductor structure beingprocessed in such a microlithography process using the portion of themask shown in FIG. 3A.

FIG. 3C is a graph illustrating an electric field that may be present atthe semiconductor structure being processed in such a microlithographyprocess using the portion of the mask shown in FIG. 3A during amicrolithography process.

FIG. 3D is a plot of the phase of the electric field of FIG. 3C that maybe present at the semiconductor structure being processed in such amicrolithography process using the portion of the mask shown in FIG. 3Aduring a microlithography process.

FIG. 4A illustrates intensity levels of electromagnetic radiation thatmay be present on a semiconductor structure being processed in amicrolithography process using a mask similar to that of FIGS. 3A and3B, but without any image reversal assist features, at nominal focusconditions and at defocused conditions.

FIG. 4B illustrates intensity levels of electromagnetic radiation thatmay be present on a semiconductor structure being processed in amicrolithography process using the mask of FIGS. 3A and 3B, whichincludes image reversal assist features, at nominal focus conditions andat defocused conditions.

FIG. 5 is a simplified plan view illustration of a portion of anotherembodiment of a microlithography mask of the invention that includesimage reversal assist features.

FIG. 6A illustrates two-dimensional areas that may be resolved on asemiconductor structure being processed in a microlithography processusing a mask similar to that of FIG. 5, but without any image reversalassist features, at nominal focus conditions and at defocused conditions(the mask being overlaid over the resolved two-dimensional areas tofacilitate illustration).

FIG. 6B illustrates two-dimensional areas that may be resolved on asemiconductor structure being processed in a microlithography processusing the mask of FIG. 5, which includes image reversal assist features,at nominal focus conditions and at defocused conditions (the mask beingoverlaid over the resolved two-dimensional areas to facilitateillustration).

FIG. 7 is a simplified plan view illustration of a portion of anotherembodiment of a microlithography mask of the invention that includesimage reversal assist features.

FIG. 8 is a simplified illustration of a portion of another embodimentof a microlithography mask of the invention that includes image reversalassist features.

DETAILED DESCRIPTION

The illustrations presented herein are not meant to be actual views ofany particular semiconductor device, transistor, or system, but aremerely idealized representations that are employed to describe thepresent invention. Additionally, elements common between figures mayretain the same numerical designation.

As used herein, the term “microlithography” means a process in which oneor more material properties of selected regions of a material arealtered by exposing the selected regions of the material toelectromagnetic radiation without exposing other regions of the materialto the electromagnetic radiation.

As used herein, the term “mask” means a patterned mask for use in amicrolithography process, through which electromagnetic radiation ispassed to expose selected regions of a material to the electromagneticradiation and to shield other selected regions of the material from theelectromagnetic radiation. Masks are often referred to in the art as“reticles,” and, as used herein, the term “mask” means and includes whatare referred to in the art as reticles.

As used herein, the term “feature,” when used with respect to a featureof a mask, means a finite area of the mask that is configured to allowelectromagnetic radiation to pass through the feature and onto asemiconductor structure to be processed using the mask, to shield thesemiconductor structure to be processed using the mask from theelectromagnetic radiation, or to shift a phase of, and/or attenuate,electromagnetic radiation passing through the mask and onto thesemiconductor structure to be processed using the mask. As one example,a mask may comprise a relatively non-transparent material, such as asemi-transparent or non-transparent material (e.g., opaque), wherein theterm “relatively” means relative to a substantially transparent material(e.g., substrate) over which the relatively transparent material isformed, and the features of such a mask might include apertures (e.g.,holes) formed through a layer of relatively non-transparent materialover the substantially transparent substrate. As another example,features of such a mask may comprise finite two-dimensional areas of therelatively non-transparent material over the transparent substrate.

As used herein, the term “primary feature,” when used with respect to afeature of a mask, means a feature that corresponds to and has a shapelike that of a feature to be patterned (e.g., resolved) on asemiconductor structure to be processed using the mask in amicrolithography process.

As used herein, the term “light feature,” when used with respect to afeature of a mask, means a feature comprising a substantially opticallytransparent area of the mask.

As used herein, the term “dark feature,” when used with respect to afeature of a mask, means a feature comprising a relativelynon-transparent area of the mask.

As used herein, the term “assist feature,” when used with respect to afeature of a mask, means a feature such as one that has a size or shapetoo small to be patterned (e.g., resolved) on a semiconductor structureto be processed using the mask in a microlithography process, but thathas a size, shape, and/or location configured to improve the contrastand/or definition of primary features to be patterned on thesemiconductor structure to be processed using the mask in amicrolithography process.

As used herein, the term “standard assist feature” means an assistfeature that has a size, shape, and/or location configured such thatelectromagnetic radiation passing through the assist feature will eitherresonate or anti-resonate with electromagnetic radiation passing throughprimary features of the mask.

As used herein, the term “image reversal assist feature” means an assistfeature that is not a standard assist feature, such that electromagneticradiation passing through the image reversal assist feature will notresonate or anti-resonate with electromagnetic radiation passing throughprimary features of the mask.

FIG. 1 is a diagrammatic block view of an embodiment of amicrolithography system 10 of the invention. The system 10 includes anillumination system 12 that is configured to emit illuminationelectromagnetic radiation 14 that may be used for microlithography.Accordingly, the system 10 may include illumination sources that areoperable to generate radiation 14 in the deep ultraviolet (DUV) portionof the spectrum, such as an excimer laser. Suitable excimer lasersources may include a xenon-fluoride (XeF) device that emits radiationat a wavelength of 351 nanometers (nm), a xenon-chloride (XeCl) devicethat emits radiation at a wavelength of 308 nm, a krypton-fluoride (KrF)device that emits radiation at a wavelength of 248 nanometers (nm), andan argon-fluoride (ArF) that emits radiation at a 193 nm wavelength.Other wavelengths also may be used.

The illumination system 12 may also include devices that provideoff-axis illumination corresponding to a selected illumination mode. Forexample, suitable devices may be configured to generate a dipole-typeillumination mode, an annular illumination mode, a two pole illuminationmode, a four pole illumination mode, or another illumination mode. Theillumination system 12 may also include other optical devices that areoperable to produce illumination radiation 14 having a desired intensityand/or distribution.

The illumination system 12 may be suitably positioned relative to animaging mask 40 (FIGS. 2A and 2B), so that the illumination radiation 14emitted from the system 12 is projected onto and through the imagingmask 40. The imaging electromagnetic illumination 18 corresponds to theradiation 14 that is selectively transmitted through the imaging mask40.

Although the mask 40 is described in further detail below with referenceto FIGS. 2A and 2B, the imaging mask 40 may include a substratecomprising a material that is substantially transparent to theillumination radiation 14, and a material over a surface of thesubstrate that is relatively non-transparent material (e.g., opaque) tothe illumination radiation 14. The substrate may comprise a materialsuch as, for example, fused quartz, soda-lime glass, borosilicate glass,or borophosphosilicate glass. The relatively non-transparent materialmay comprise, for example, at least one of a metal (e.g., chromium), ametal alloy (e.g., a chromium-based alloy), and a material comprisingmolybdenum and/or silicon (e.g., molybdenum silicide). The relativelynon-transparent material also may comprise oxygen and nitrogen atoms aswell. In some embodiments, relatively non-transparent materials may beused to provide phase shifting areas on the mask 40 for shifting a phaseof the illumination radiation 14 as it passes through the mask 40 andbecomes the imaging radiation 18.

With continued reference to FIG. 1, the microlithography system 10 mayfurther include a projection lens assembly 20 that is configured toproject the imaging illumination onto a workpiece. For example, theprojection lens assembly might be configured to collect the imagingillumination 18 and form focused imaging electromagnetic radiation 21therefrom. The focused imaging radiation 21 may then be directed onto aworkpiece 22 being processed to form a semiconductor device. By way ofexample and not limitation, the projection lens assembly 20 may beconfigured to focus the imaging radiation 21 such that the patternedimage at the surface of the workpiece 22 is about one-fourth the size ofthe mask 40. In other words, the mask 40 may be about four times aslarge as the resulting image formed on the surface of the workpiece 22.Thus, features on the mask 40 may have dimensions that are about fourtimes as large as the features to be patterned at the surface of theworkpiece 22.

The workpiece 22 may comprise a photoresist material 24 that is disposedover a semiconductor structure 26 (e.g., a die or wafer). Thephotoresist material 24 is responsive to the imaging radiation 21. Sincethe photoresist 24 may be repetitively exposed to the focused imagingradiation 21 to form separately exposed areas on the semiconductorstructure 26, the workpiece 22 may be positioned upon a stage 28 thatmay be translated in one or more of several (e.g., two or three)mutually orthogonal directions.

In some embodiments of the invention, the mask 40 may comprise anattenuated phase-shifting mask. For an improved understanding of suchembodiments, FIGS. 2A through 2C are used to illustrate how anattenuated phase shifting mask may be used to provide patternedintensity levels of electromagnetic radiation on a semiconductorstructure to be processed using an attenuated phase shifting mask in amicrolithography process.

FIGS. 2A and 2B illustrate a portion of an embodiment of amicrolithography mask 40 of the invention. FIG. 2A is a partial planview of the illustrated portion of the mask 40, and FIG. 2B is across-sectional view of the portion of the mask 40 taken along sectionline 2B-2B shown in FIG. 2A. As shown in FIG. 2A, the mask 40 includes asubstantially transparent substrate 42 and relatively non-transparentmaterial 44 on selected areas of the substrate 42, thus defining aplurality of light features (illustrated as clear areas in FIG. 2A) anddark features (illustrated as stippled areas in FIG. 2A) on the mask 40.In some embodiments, the mask 40 may comprise an attenuated phaseshifting mask, wherein the relatively non-transparent material 44 is anabsorbing π (pi or 180°) phase shifting material. Such masks may bereferred to in the art as “transmission-pi” or “t-pi” masks. By way ofexample and not limitation, the transmission of the relativelynon-transparent material 44 may be about 10% or less. In suchembodiments, the dark features are configured (e.g., sized and composed)to attenuate and shift a phase of electromagnetic radiation passingthrough the relatively non-transparent material 44 of the dark featuresof the mask 40.

The portion of the mask 40 shown in FIGS. 2A and 2B is configured topattern (e.g., resolve) two sets of three lines on a workpiece 22 whenthe mask 40 is used in a microlithography process. In particular, theportion of the mask 40 includes primary line features 50A through 50F,each of which includes a line. Thus, the portion of the mask 40 shown inFIGS. 2A and 2B is configured to pattern portions of six lines in aphotoresist material 24 on a workpiece 22 being processed using the mask40.

The lines of primary features 50A through 50C are disposed in a firstgroup 52A, and the lines of primary features 50D through 50F aredisposed in a second group 52B, as shown in FIG. 2A. The primaryfeatures in each group 52A, 52B may be densely packed relative to oneanother at a pitch close to the critical dimension (CD) of themicrolithography process. The spacing between the groups 52A, 52B may besomewhat greater than the critical dimension of the microlithographyprocess. Thus, the primary features 50D through 50F may comprise whatare referred to in the art as semi-isolated features, such as thosecommonly found in the laterally peripheral areas of the integratedcircuits of semiconductor devices (e.g., memory devices and logicdevices).

The mask 40 also includes various assist features that are configured toimprove the contrast of the patterned image and/or the definition of thesix lines to be patterned in a photoresist material 24 on a workpiece 22by the primary features 50A through 50F.

The mask 40 includes a plurality of standard assist features 54, each ofwhich may be located at resonant or anti-resonant locations relative tothe primary features 50A through 50F. In particular, the standard assistfeatures 54 may be located to the lateral sides of each of the groups52A, 52B of primary features 50A through 50F, as shown in FIGS. 2A and2B. The standard assist features 54 may comprise light features, each ofwhich includes a line having a shape and configuration similar to thatof a line of the primary features 50A through 50F, but having a smallerwidth that is below the resolution limit. The standard assist features54 may serve as light field assist features, in that they may serve toincrease exposure to imaging radiation in the regions of photoresistmaterial 24 corresponding to the primary features to be patterned in thephotoresist material 24 on a workpiece 22 being processed using the mask40. Thus, the standard assist features 54 may assist in improving thedefinition of the primary features to be patterned in a photoresistmaterial 24 on a workpiece 22 being processed using the mask 40, but maynot themselves be used to directly pattern a photoresist material 24 ona workpiece 22 being processed using the mask 40.

The primary features 50A through 50F in each of the groups 52A, 52B maythemselves be densely packed, such that there is no resonant oranti-resonant position between any of the primary features 50A through50F within any one of the groups 52A, 52B, such that a standard assistfeature could be placed therebetween.

As shown in FIGS. 2A and 2B, the mask 40 also includes a plurality ofimage reversal assist features 56, each of which may be located otherthan at a resonant or anti-resonant location relative to the primaryfeatures 50A through 50F. In particular, the image reversal assistfeatures 56 may be located between the primary features 50A through 50Fwithin each of the groups 52A, 52B of primary features 50A through 50F,as shown in FIGS. 2A and 2B. The image reversal assist features 56, likethe standard assist features 54, also may comprise light features, eachof which includes a line having a shape and configuration similar tothat of a line of the primary features 50A through 50F, but having asmaller width that is below the resolution limit. Due at least partiallyto their location, however, the image reversal assist features 56 mayserve as dark field assist features, instead of as light field assistfeatures. In other words, the image reversal assist features 56 mayserve to decrease undesirable exposure to imaging radiation in theregions of photoresist material 24 between those corresponding to theprimary features to be patterned in the photoresist material 24 on aworkpiece 22 being processed using the mask 40. Thus, in this manner,the image reversal assist features 56 further assist in improving thedefinition of the primary features to be patterned in a photoresistmaterial 24 on a workpiece 22 being processed using the mask 40, but maynot themselves be used to directly pattern a photoresist material 24 ona workpiece 22 being processed using the mask 40.

FIGS. 3A through 3D are used to illustrate how image reversal assistfeatures 56 may be positioned on a mask 40. FIG. 3A is an enlarged viewof a portion of FIG. 2B that includes the primary features 50A through50C and the image reversal assist features 56 therebetween. FIGS. 3Bthrough 3D are graphs that are vertically aligned with the mask 40 toillustrate the effect the various regions of the illustrated portion ofthe mask 40 have on the resulting imaging electromagnetic radiationultimately impinging on a workpiece, like the workpiece 22 of FIG. 1.

FIG. 3B is a graph illustrating an intensity of electromagneticradiation that may be present at a surface of a workpiece 22 beingprocessed in a microlithography process using the portion of the mask 40of FIG. 3A. As shown in FIG. 3B, the intensity plot is sinusoidal inshape and varies between a maximum intensity and a minimum intensity,with peaks corresponding to the centers of the primary features 50Athrough 50C of the mask 40. The intensity of radiation at a surface of aworkpiece 22 being processed in a microlithography process isproportional to the square of the electric field present at the surfaceof the workpiece 22. As a result, the intensity is always positive.

FIG. 3C is a graph illustrating an electric field that may be present ata surface of a workpiece 22 being processed in a microlithographyprocess using the portion of the mask 40 of FIG. 3A. As shown in FIG.3C, the electric field plot is also sinusoidal in shape and variesbetween maximum positive (e.g., “0”) phase value and maximum negative(e.g., “π”) phase value (which correspond to the minimum/lowest pointsin the graph). The alternating peaks and valleys of the plot of theelectric field shown in FIG. 3C correspond to the peaks of the plot ofthe radiation intensity shown in FIG. 3B, and, hence, to the centers ofthe primary features 50A through 50C of the mask 40.

FIG. 3D is a plot of the phase of the electrical field shown in FIG. 3Cthat may be present at a surface of a workpiece 22 being processed in amicrolithography process using the portion of the mask 40 of FIG. 3A. Asshown in FIG. 3D, the transitions between the positive phase regions andthe negative phase regions are immediate, step-wise transitions.

As will be appreciated upon comparison of FIGS. 3A, 3C, and 3D, theimage reversal assist features 56 may be provided at, or at leastproximate, locations on the mask 40 between the primary features 50Athrough 50F that correspond to the locations at which the phase of theelectrical field present at the surface of the workpiece 22 beingprocessed using the mask 40 transitions between the “0” phase values andthe “π” phase values. As the primary features 50A through 50F aredensely packed features, there are no locations between any two of theprimary features 50A through 50F that correspond to resonant oranti-resonant locations with the primary features 50A through 50F. Thus,there is no space for standard assist features 54 between any two of theprimary features 50A through 50F.

Referring again to FIGS. 2A and 2B, as one non-limiting embodiment setforth merely as an example, each of the primary features 50A through 50Fmay have an average width of about three hundred and eight nanometers(308 nm) on the mask 40, such that the primary features to be patternedin the photoresist material 24 at a surface of a workpiece 22 have anaverage width of about seventy-seven nanometers (77 nm), which isone-fourth of the width of the primary features 50A through 50F on themask 40. The pitch of the primary features 50A through 50F on the mask40 also may be about three hundred and eight nanometers (308 nm), suchthat the center of one of the primary features 50A through 50F isseparated from the center of the next immediately adjacent primaryfeatures 50A through 50F by about three hundred and eight nanometers(308 nm). The minimum feature size (i.e., the critical dimension) insuch an embodiment may be about fifty-seven nanometers (57 nm). Each ofthe image reversal assist features 56 may have an average width ofeighty nanometers (80 nm) on the mask 40, such that, after reduction bythe projection lens assembly, the lines in the image applied to thephotoresist material 24 at the surface of the workpiece 22 have anaverage width of about twenty nanometers (20 nm), which is below theminimum feature size, and, thus, will not be patterned in thephotoresist material 24.

In additional embodiments, each of the primary features 50A through 50Fmay have an average width of less than about four hundred nanometers(400 nm) on the mask 40, less than about three hundred nanometers (300nm) on the mask 40, less than about two hundred nanometers (200 nm) onthe mask 40, or even than about one hundred nanometers (100 nm) on themask 40. In some embodiments, the distance between the primary features50A through 50F in each respective group 52A, 52B thereof may be betweenabout 90% and about 110% of the average width of the primary features50A through 50F. In additions embodiments, the distance between theprimary features 50A through 50F in each respective group 52A, 52Bthereof may be up to about 150% of the average width of the primaryfeatures 50A through 50F. Furthermore, in some embodiments, the imagereversal assist features 56 may have an average width that is betweenabout 10% and about 70% (e.g., about 25%) of the average width of theprimary features 50A through 50F, and that is below the criticaldimension of the microlithography process in which the mask 40 is to beemployed.

As known in the art, the ability to clearly pattern an image in aphotoresist material 24 using a microlithography system 10 like that ofFIG. 1 is limited by the wavelength of the electromagnetic radiationemployed in the system 10 and the ability of the mask 40 and theprojection lens assembly 20 to capture diffraction orders of theradiation and focus the radiation onto the workpiece 22. The minimumfeature size that a microlithography system 10 can pattern may beapproximated using Equation 1 below:CD=k ₁(λ/NA)  Equation 1wherein CD is the critical dimension (i.e., the minimum feature sizethat can be patterned), k₁ is a coefficient that encapsulates processrelated factors, and NA is the numerical aperture of the projection lensassembly as seen by the workpiece 22. Thus, in order to reduce thecritical dimension, “low k₁” processes are required (e.g., processeshaving a k₁ value of about 0.30 or less). In low k1 microlithographyprocesses, however, the illumination radiation is often optimized towardthe critical, densely packed features of the semiconductor device beingfabricated, which may leave the peripheral isolated and/or semi-isolatedfeatures un-optimized. Thus, a sufficient depth of focus should bemaintained to ensure that all features to be patterned, includingdensely packed central features, as well as isolated and semi-isolatedperipheral features. By employing image reversal assist features 56 inmasks 40 used in low k₁ microlithography processes as described herein,the depth of focus may be improved relative to processes in whichsimilar masks not including such image reversal assist features 56 areemployed.

FIGS. 4A and 4B are used to illustrate how use of masks having imagereversal assist features 56 may improve the depth of focus relative toprocesses in which similar masks not including such image reversalassist features 56 are employed.

FIG. 4A illustrates the results of computer simulations conducted usinga computer model of a portion of a mask similar to the mask 40 of FIGS.2A and 2B and including a group 52A of three primary features 50Athrough 50C and standard assist features 54 to the lateral sides of thegroup 52A, but that does not include image reversal assist feature 56between the primary features 50A through 50C in the group 52A. Thecomputer simulations were conducted using Technology Computer AidedDesign (TCAD) Sentaurus Lithography (S-Litho) software, which iscommercially available from Synopsys, Inc. of Mountain View, Calif. FIG.4A includes two graphs, one of which illustrates the calculatedintensity of the electromagnetic radiation that would be present at asurface of a workpiece 22 being processed in a microlithography processusing the portion of the mask of FIG. 4A (which does not include imagereversal assist features 56) at nominal focus conditions, the other ofwhich illustrates the calculated intensity of the electromagneticradiation that would be present at a surface of a workpiece 22 beingprocessed in a microlithography process using the portion of the mask ofFIG. 4A at defocused conditions. The horizontal dashed line in each ofthe graphs illustrates the intensity level at which the photoresistmaterial 24 will become “exposed” to the radiation and undergo thephysical and/or chemical changes associated with exposure. As shown inthe graph of FIG. 4A corresponding to the defocus conditions, there isinsufficient contrast between the intensity peaks corresponding to theprimary features 50A through 50C to prevent exposure of the photoresistmaterial 24 between the peaks. Thus, a single line having a relativelylarger width W would likely be patterned in the photoresist material 24under such defocus conditions.

FIG. 4B illustrates the results of computer simulations conducted usinga computer model of a portion of the mask 40 of FIGS. 2A and 2Bincluding the group 52A of three primary features 50A through 50C, thestandard assist features 54 to the lateral sides of the group 52A, andthe image reversal assist features 56 between the primary features 50Athrough 50C in the group 52A. FIG. 4B also includes two graphs, one ofwhich illustrates the calculated intensity of the electromagneticradiation that would be present at a surface of a workpiece 22 beingprocessed in a microlithography process using the illustrated portion ofthe mask 40 shown in FIG. 4B (which does include image reversal assistfeatures 56) at nominal focus conditions, and the other of whichillustrates the calculated intensity of the electromagnetic radiationthat would be present at a surface of a workpiece 22 being processed ina microlithography process using the portion of the mask 40 at the samedefocus conditions used in the defocus simulations associated with FIG.4A. Again, the horizontal dashed line in each of the graphs illustratesthe intensity level at which the photoresist material 24 will become“exposed” to the radiation and undergo the physical and/or chemicalchanges associated with exposure. As shown in the graph of FIG. 4Bcorresponding to the defocus conditions, there is sufficient contrastbetween the intensity peaks corresponding to the primary features 50Athrough 50C at the same defocus conditions associated with the defocusconditions of FIG. 4A to prevent exposure of the photoresist material 24between the peaks. Thus, three separate lines having relatively smallerwidths would likely be patterned, as intended, in the photoresistmaterial 24 under such defocus conditions.

Thus, by employing the image reversal assist features 56 in the mask 40,the depth of focus may be improved relative to processes in whichsimilar masks not including such image reversal assist features 56 areemployed.

FIG. 5 is a simplified plan view illustration of a portion of anotherembodiment of a microlithography mask 60 of the invention that includesimage reversal assist features 76. The mask 60 is generally similar tothe mask 40 of FIGS. 2A and 2B and includes a substantially transparentsubstrate 62 and relatively non-transparent material 64 on selectedareas of the substrate 62, thus defining a plurality of light features(illustrated as clear areas in FIG. 5) and dark features (illustrated asstippled areas in FIG. 5) on the mask 60. In some embodiments, the mask60 may comprise an attenuated phase shifting mask, wherein therelatively non-transparent material 64 is an absorbing π (pi or 180°)phase shifting material. The portion of the mask 60 shown in FIG. 5 isconfigured to pattern (e.g., resolve) a set of three primary features ona workpiece 22 when the mask 60 is used in a microlithography process toprocess the workpiece 22. In particular, the portion of the mask 60includes a group 72 of primary features 70A through 70C. The primaryfeatures 70A and 70C are line features, each of which includes anintegral contact pad area 78 (which may or may not be symmetrical and ofsimilar shape). The primary feature 70B is a line feature, similar tothe primary features 50A-50F of FIGS. 2A and 2B, that does not includean integral contact pad area 78 (at least in the vicinity of the contactpad areas 78 of the primary features 70A and 70C). Thus, the portion ofthe mask 60 shown in FIG. 5 is configured to pattern portions of threelines, two of which also having an integrated contact pad area, in aphotoresist material 24 on a workpiece 22 being processed using the mask60.

The primary features 70A through 70C may comprise semi-isolatedfeatures, such as those commonly found in the laterally peripheral areasof the integrated circuits of semiconductor devices (e.g., memorydevices and logic devices), as previously discussed in relation to FIGS.2A and 2B.

The mask 60 also includes various assist features that are configured toimprove the contrast of the patterned image and/or the definition of theprimary features to be printed in a photoresist material 24 on aworkpiece 22 by the primary features 70A through 70C.

The mask 60 includes a plurality of standard assist features 74, each ofwhich may be located at resonant or anti-resonant locations relative tothe primary features 70A through 70C. In particular, the standard assistfeatures 74 may be located to the lateral sides of the group 72 ofprimary features 70A through 70C, as shown in FIG. 5. The standardassist features 74 may comprise light features having widths that arebelow the resolution limit for features on the mask 60. The standardassist features 74 may serve as light field assist features, in thatthey may serve to increase exposure to imaging radiation in the regionsof photoresist material 24 corresponding to the primary features to bepatterned in the photoresist material 24 on a workpiece 22 beingprocessed using the mask 60. Thus, the standard assist features 74 mayassist in improving the definition of the primary features to bepatterned in a photoresist material 24 on a workpiece 22 being processedusing the mask 60, but may not themselves be used to directly pattern aphotoresist material 24 on a workpiece 22 being processed using the mask60.

The mask 60 also includes dark standard features 79, which may be sized,shaped, and/or located to assist in the definition of contact area padsto be patterned primarily by the contact pad areas 78 of the mask 60.The dark standard features 79 may be sized, shaped, and located suchthat the light intensity in the contact pad areas 78 will be reducedwhile maintaining sufficient contrast to prevent the light at thecontact pad areas 78 from being blurred significantly at defocusconditions in order to maintain the presence of photoresist materialbetween the contact pad areas 78 and the primary feature 70B. While thedark standard features 79 themselves might unintentionally print atdefocus conditions, the image reversal assist features 76 may alleviatethis issue because, while the image reversal assist features 76 mayreduce the light intensity at photoresist material between the contactpad areas 78 and the primary feature 70B, they also may expel lighttoward the lateral sides of the image reversal assist features 76 andinto the contact pad areas 78, such that the standard dark features 79are less likely to print. The increased light intensity at the contactpad areas 78 may result from the image reversal assist features 76actually transmitting more light at the mask relative to the case (FIG.6A) in which the image reversal assist features 76 are absent (FIG. 6A).

The primary features 70A through 70C in the group 72 may themselves bedensely packed, such that there is no resonant or anti-resonant positionbetween any of the primary features 70A through 70C within the group 72,such that a standard assist feature could be placed therebetween.

As shown in FIG. 5, the mask 60 also includes a plurality of imagereversal assist features 76, each of which may be located other than ata resonant or anti-resonant location relative to the primary features70A through 70C. In particular, the image reversal assist features 76may be located between the primary features 70A through 70C, as shown inFIG. 5. The image reversal assist features 76, like the standard assistfeatures 74, also may comprise light features, each of which includes aline having a shape and configuration similar to that of a line of theprimary features 70A through 70C, but having a smaller width that isbelow the resolution limit of features on the mask 60. The imagereversal assist features 76 may not have any feature corresponding tothe contact pad areas 78 of the primary features 70A through 70C, otherthan the area of the contact pad areas 78 provided by the lines of theprimary features 70A through 70C. Due at least partially to theirlocation, however, the image reversal assist features 76 may serve asdark field assist features, instead of as light field assist features,as do the standard assist features 74. In other words, the imagereversal assist features 76 may serve to decrease undesirable exposureto imaging radiation in the regions of photoresist material 24 betweenthose corresponding to the primary features to be patterned in thephotoresist material 24 on a workpiece 22 being processed using the mask60. Thus, in this manner, the image reversal assist features 76 furtherassist in improving the definition of the primary features to bepatterned in a photoresist material 24 on a workpiece 22 being processedusing the mask 60, but may not themselves be used to directly pattern ina photoresist material 24 on a workpiece 22 being processed using themask 60.

FIGS. 6A and 6B are used to illustrate how use of masks having imagereversal assist features 76 may improve the depth of focus relative toprocesses in which similar masks not including such image reversalassist features 76 are employed.

FIG. 6A illustrates the results of computer simulations conducted usinga computer model of a portion of a mask similar to the mask 60 of FIG. 5and including a group 72 of three primary features 70A through 70C andstandard assist features 74 to the lateral sides of the group 72, butthat does not include image reversal assist feature 76 between theprimary features 70A through 70C. FIG. 6A includes two aerial images,one of which illustrates the calculated areas (the stippled areas inFIG. 6A) that would be patterned in photoresist material 24 at a surfaceof a workpiece 22 being processed in a microlithography process usingthe portion of the mask of FIG. 6A (which does not include imagereversal assist features 76) at nominal focus conditions, the other ofwhich illustrates the calculated areas (the stippled areas in FIG. 6A)that would be patterned in photoresist material 24 at a surface of aworkpiece 22 being processed in a microlithography process using theportion of the mask of FIG. 6A at defocused conditions. As shown in theaerial image of FIG. 6A corresponding to the defocus conditions, thereis insufficient definition of the contact pad areas to be patterned bythe contact pad areas 78 of the primary features 70A and 70C of the mask60. Thus, contact pad areas formed may not be continuous with, and,hence, may not be conductively coupled to the lines of primary featuresto be patterned by the primary features 70A and 70C. As a result, theconductive pad areas may be defective and unsuitable for use ifpatterned under such defocus conditions.

FIG. 6B illustrates the results of computer simulations conducted usinga computer model of a portion of the mask 60 of FIG. 5 including thegroup 72 of three primary features 70A through 70C, the standard assistfeatures 74 to the lateral sides of the primary features 70A through70C, and the image reversal assist features 76 between the primaryfeatures 70A through 70C. FIG. 6B also includes two aerial images, oneof which illustrates the calculated areas (the stippled areas in FIG.6B) that would be patterned in photoresist material 24 at a surface of aworkpiece 22 being processed in a microlithography process using theportion of the mask 60 of FIG. 6B (which does include image reversalassist features 76) at nominal focus conditions, and the other of whichillustrates the calculated areas (the stippled areas in FIG. 6B) thatwould be patterned in photoresist material 24 at a surface of aworkpiece 22 being processed in a microlithography process using theportion of the mask 60 of FIG. 6B (which does include image reversalassist features 76) at the same defocus conditions used in the defocussimulations associated with FIG. 6A. As shown in the aerial image ofFIG. 6B corresponding to the defocus conditions, there is sufficientdefinition of the contact pad areas to be patterned by the contact padareas 78 of the primary features 70A and 70C of the mask 60 at the samedefocus conditions associated with the defocus conditions of FIG. 6A toresult in the formation of acceptable contact pad areas.

Thus, by employing the image reversal assist features 76 in the mask 60,the depth of focus may be improved relative to processes in whichsimilar masks not including such image reversal assist features 76 areemployed.

As described hereinabove with reference to FIGS. 3A through 3D, inaccordance with some embodiments of the invention, a single imagereversal assist feature may be provided at, or at least proximate, oneor more locations on a microlithography mask between primary featuresthat corresponds to a locations at which the phase of the electricalfield present at the surface of the workpiece 22 being processed usingthe mask transitions between a “0” phase value and a “π” phase value. Inaccordance with additional embodiments of the invention, two or moreimage reversal assist features may be provided on a microlithographymask between primary features, and the two or more image reversal assistfeatures may be centered about a location at, or at least proximate, alocation that corresponds to a location at the surface of the workpiece22 being processed using the mask at which the phase of the electricalfield present at the surface transitions between a “0” phase value and a“π” phase value.

For example, FIG. 7 is a simplified plan view illustration of a portionof another embodiment of a microlithography mask 80 of the inventionthat includes image reversal assist features 96. The mask 80 isgenerally similar to the mask 40 of FIGS. 2A and 2B and includessubstantially transparent substrate 82 and relatively non-transparentmaterial 84 on selected areas of the substrate 82, thus defining aplurality of light features (illustrated as clear areas in FIG. 7) anddark features (illustrated as stippled areas in FIG. 7) on the mask 80.The mask 80 includes a group 92 of primary features 90A through 90C, aswell as standard assist features 94, which may be at least substantiallysimilar to the group 72A of primary features 70A through 70C and thestandard assist features 54, respectively, of the mask 40 of FIGS. 2Aand 2B.

The mask 80, however, includes a first pair of image reversal assistfeatures 96 between first primary feature 70A and the second primaryfeature 70B, and a second pair of image reversal assist features 96between the second primary feature 70B and the third primary feature70C. The pairs of image reversal assist features 96 are located otherthan at resonant or anti-resonant locations relative to the primaryfeatures 90A through 90C. In particular, each pair of image reversalassist features 96 may be located between two of the primary features90A through 90C, as shown in FIG. 7. Each pair of image reversal assistfeatures 96 may be centered about a location at, or at least proximateto, a location that corresponds to a location at the surface of theworkpiece 22 being processed using the mask 80 at which the phase of theelectrical field present at the surface transitions between a “0” phasevalue and a “π” phase value (as shown in FIG. 3D). In additionalembodiments, a group of three, four, or more image reversal assistfeatures 96 may be centered about a location at, or at least proximateto, one or more such locations on the mask 80.

The image reversal assist features 96, like the standard assist features94, may comprise light features, each of which includes a line having ashape and configuration similar to that of a line of the primaryfeatures 90A through 90C, but having a smaller width that is below theresolution limit of features on the mask 80. Due at least partially totheir location, however, the pairs of image reversal assist features 96may serve as dark field assist features, instead of as light fieldassist features, as do the standard assist features 94. In other words,the image reversal assist features 96 may serve to decrease undesirableexposure to imaging radiation in the regions of photoresist material 24between those corresponding to the primary features to be patterned inthe photoresist material 24 on a workpiece 22 being processed using themask 80. Thus, in this manner, the image reversal assist features 96further assist in improving the definition of the primary features to bepatterned in a photoresist material 24 on a workpiece 22 being processedusing the mask 80, but may not themselves be used to directly pattern aphotoresist material 24 on a workpiece 22 being processed using the mask80.

The image reversal assist features 96 may serve to enhance the depth offocus of the mask 80 in a similar manner to that provided by the imagereversal assist features 56 of the mask 40 of FIGS. 2A and 2B and theimage reversal assist features 76 of the mask 60 of FIG. 5, as discussedabove.

Additional embodiments of the invention include methods of formingmicrolithography masks as described herein, such as, for example, themask 40 of FIGS. 2A and 2B, the mask 60 of FIG. 5, and the mask 80 ofFIG. 7. As discussed in further detail below, a relativelynon-transparent material on a mask substrate may be patterned to form atleast one image reversal assist feature, which may be disposed betweenat least two primary features.

FIG. 8 is a diagrammatic block view of a mask exposure system 100 thatmay be used to form microlithography masks as described herein. The maskexposure system 100 may be used to fabricate a microlithography maskthat includes image reversal assist features, as described herein. As anexample, a mask blank 106 may be provided that includes one or moresurface coatings 108 thereon. For example, the surface coatings 108 mayinclude a layer of relatively non-transparent material (e.g., opaque)that ultimately may form dark field regions of the mask to be formedfrom the blank. The layer of relatively non-transparent material may beselectively patterned to form a microlithography mask as describedherein. As discussed above, the relatively non-transparent material maycomprise a phase shift material, such that patterning the relativelynon-transparent material comprises patterning a phase shift material onthe mask substrate.

To pattern the layer of relatively non-transparent material, a patternedmask layer may be formed over the relatively non-transparent material onthe mask substrate, and regions of the relatively non-transparentmaterial exposed through the patterned mask layer may be etched. Thelayer of relatively non-transparent material may be wet or dry etchedthrough apertures of the patterned mask layer to form the selectedpattern in the relatively non-transparent material. Such a mask layermay be formed using a resist material. A resist material may bedeposited over the relatively non-transparent material. Selected regionsof the resist material may be exposed to energy to alter a property ofthe resist material in the selected regions. The resist material thenmay be developed to remove one of exposed and unexposed regions of theresist material. Thus, the surface coatings 108 shown in FIG. 8 also mayinclude a layer of resist material disposed over the layer ofsemi-transparent or non-transparent material.

Since the microlithography masks disclosed herein (and to be formedusing the mask exposure system 100) include assist features that are ofsuch small size that they may not be resolved using standardphotolithography systems that employ electromagnetic radiation toselectively expose a photoresist material, other types of lithographysystems having lower resolution limits may be employed to expose aresist material and pattern a mask layer. For example, the mask exposuresystem 100 may comprise an electron beam lithography system.

Thus, the mask exposure system 100 may include an electron beam system102 that projects an electron beam 104 towards a microlithography maskblank 106, which may have at least one surface coating 108 thereon. Asdiscussed above, the surface coating 108 may comprise an electron-beamresist material. Although not shown in FIG. 8, the electron beam system102 may also include a beam source (such as a thermionic source or acold cathode source), a blanker that is configured to interrupt the beamsource, a magnetic beam deflection system, and/or an electrostaticdeflection system that is configured to steer the electron beam 104across a selected portion of the surface of the mask blank 106. Sincethe magnetic beam deflection system and/or the electrostatic deflectionsystem may be unable to steer the electron beam 104 to all locations onthe surface of the mask blank 106, the mask blank 106 may be positionedupon a stage 110 that may be translated in one or more of several (e.g.,three) mutually-orthogonal directions. The system 100 may also include avacuum chamber 112 that substantially encloses the mask blank 106 andthe stage 110, so that the electron beam 104 may be directed from theelectron beam system 102 to the mask blank 106 under vacuum.

The system 100 also includes a control system 113, which may include acomputer device comprising memory 114 and a processor 115. A computerprogram may reside in the memory 114 of the control system 113. A modelof a mask to be fabricated using the mask exposure system 100 also maybe stored in the memory 114 of the control system 113, and the computerprogram may be configured to control the various active components ofthe mask exposure system 100 to cause the mask exposure system 100 toform a mask from the mask blank 106. As non-limiting examples, thecontrol system 113 may include various known devices such as a mainframecomputer device, desktop computer device, a portable or “laptop”computer device, a programmable logic controller, or a custom builtcomputer device.

Still referring to FIG. 8, the mask exposure system 100 may include aninterface 116 coupled to the control system 113 and configured toreceive instructions from the control system 113. The interface 116 maybe configured to translate instructions received from the control system113 and convert the received instructions into signals that may be usedto control the active components of the mask exposure system 100, suchas, for example, the electron beam system 102 and the stage 110.Accordingly, the control system 113 and the interface 116 maycooperatively control scanning of the electron beam 104 across thesurface of the imaging mask blank 106. Suitable scanning methods mayinclude raster-scanning and vector scanning the electron beam 104 overthe mask blank 106.

After exposing selected regions of the photoresist material to theelectron beam 104, the photoresist material may be developed to form apatterned mask layer. An etching apparatus (not shown in FIG. 8) thenmay be used to selectively etch (wet or dry) the layer ofsemi-transparent or non-transparent material to selectively pattern thesemi-transparent or non-transparent material and form a microlithographymask as described herein.

Thus, in some embodiments, the invention includes microlithography masksthat include a substrate and a patterned non-transparent orsemi-transparent material on the substrate. The non-transparent orsemi-transparent material defines a plurality of features of the maskthat include at least one image reversal assist feature disposed betweenat least two primary features.

In additional embodiments, the invention includes microlithographysystems that include such masks. For example, a microlithography systemmay include a stage for supporting a workpiece to be processed using themicrolithography system, an illumination system for emittingelectromagnetic radiation, a mask located and configured for selectivelypatterning electromagnetic radiation emitted by the illumination system,and a projection lens assembly for projecting electromagnetic radiationemitted by the illumination system and patterned by the mask onto aworkpiece to be supported by the stage. The mask may include at leastone image reversal assist feature disposed between at least two primaryfeatures.

In yet further embodiments, the invention includes methods of formingmicrolithography masks that include patterning a non-transparent orsemi-transparent material on a mask substrate, and forming at least oneimage reversal assist feature disposed between at least two primaryfeatures.

While the present invention has been described in terms of certainillustrated embodiments and variations thereof, it will be understoodand appreciated by those of ordinary skill in the art that embodimentsof the invention are not so limited. Rather, additions, deletions andmodifications to the illustrated embodiments may be effected withoutdeparting from the scope of the invention as defined by the claims thatfollow, and their legal equivalents. Furthermore, elements and featuresof one embodiment described herein may be implemented into, or combinedwith, any other embodiment described herein without departing from thescope of the invention.

1. A microlithography mask, comprising: a substantially transparentmaterial; and a patterned, relatively non-transparent material over thesubstantially transparent material, the relatively non-transparentmaterial defining a plurality of features of the microlithography mask,the plurality of features comprising: at least two primary features; andat least one image reversal assist feature between the at least twoprimary features.
 2. The microlithography mask of claim 1, wherein thesubstantially transparent material comprises one or more of quartzglass, soda-lime glass, borosilicate glass, and borophosphosilicateglass.
 3. The microlithography mask of claim 1, wherein the relativelynon-transparent material comprises a phase shifting material.
 4. Themicrolithography mask of claim 1, wherein the relatively non-transparentmaterial comprises at least one of a metal, a metal alloy, and/ormolybdenum silicide.
 5. The microlithography mask of claim 1, whereinthe at least two primary features comprise apertures through therelatively non-transparent material.
 6. The microlithography mask ofclaim 1, wherein the at least one image reversal assist feature islocated on the microlithography mask at least proximate a locationcorresponding to a location at a surface of a workpiece to be processedusing the mask at which a phase of an electrical field present at thesurface transitions between phases.
 7. The microlithography mask ofclaim 1, wherein the at least one image reversal assist featurecomprises a plurality of image reversal assist features, the pluralityof image reversal assist features centered about a location on themicrolithography mask at least proximate to a location corresponding toa location at a surface of a workpiece to be processed using the mask atwhich a phase of an electrical field present at the surface transitionsbetween phases.
 8. The microlithography mask of claim 1, wherein each ofthe at least two primary features has an average width of less thanabout four hundred nanometers (400 nm) on the microlithography mask, andwherein the at least one image reversal assist feature has an averagewidth between about 10% and about 70% of the average width of each ofthe at least two primary features.
 9. The microlithography mask of claim8, wherein the at least one image reversal assist feature has an averagewidth of about 25% of the average width of each of the at least twoprimary features.
 10. The microlithography mask of claim 8, wherein theat least two primary features are separated by an average distance ofbetween about 90% and about 110% of the average width of each of the atleast two primary features.
 11. The microlithography mask of claim 8,further comprising at least one standard assist feature locatedproximate at least one primary feature of the at least two primaryfeatures.
 12. A microlithography system, comprising: an illuminationsystem configured to emit electromagnetic radiation; a mask comprisingat least one image reversal assist feature between at least two primaryfeatures, the mask configured to selectively transmit electromagneticradiation emitted by the illumination system; and a projection lensassembly configured to project electromagnetic radiation emitted by theillumination system and transmitted by the mask onto a workpiece. 13.The microlithography system of claim 12, wherein the illumination systemis configured to emit at least substantially monochromatic radiationhaving a wavelength at one of 193 nanometers, 248 nanometers, and 308nanometers.
 14. The microlithography system of claim 12, wherein theillumination system comprises an argon-fluoride laser configured to emitradiation at a 193 nm wavelength.
 15. The microlithography system ofclaim 12, wherein the mask comprises an attenuated phase shifting mask.16. The microlithography system of claim 15, wherein the attenuatedphase shifting mask comprises a molybdenum silicide material.
 17. Themicrolithography system of claim 12, wherein the at least one imagereversal assist feature is located on the mask at least proximate alocation corresponding to a location at a surface of a workpiece to beprocessed using the mask at which a phase of an electrical field presentat the surface transitions between a 0 phase and a π phase.
 18. Themicrolithography system of claim 12, wherein the at least one imagereversal assist feature comprises a plurality of image reversal assistfeatures, the plurality of image reversal assist features centered abouta location on the mask at least proximate to a location corresponding toa location at a surface of a workpiece to be processed using the mask atwhich a phase of an electrical field present at the surface transitionsbetween phases.
 19. The microlithography system of claim 12, whereineach of the at least two primary features has an average width of lessthan about four hundred nanometers (400 nm) on the mask, and wherein theat least one image reversal assist feature has an average width betweenabout 10% and about 70% of the average width of each of the at least twoprimary features.
 20. A method of forming a microlithography mask,comprising patterning a relatively non-transparent material on asubstantially transparent material, wherein the patterning comprisesforming at least one image reversal assist feature disposed between atleast two primary features.
 21. The method of claim 20, whereinpatterning a relatively non-transparent material on a substantiallytransparent material comprises patterning a phase shift material on amask substrate.
 22. The method of claim 20, wherein patterning arelatively non-transparent material comprises: forming a patterned masklayer over the relatively non-transparent material; and etching regionsof the relatively non-transparent material exposed through the patternedmask layer.
 23. The method of claim 22, wherein forming a patterned masklayer comprises: depositing a resist material over the relativelynon-transparent material; exposing selected regions of the resistmaterial to energy to alter a property of the resist material in theselected regions; and developing the resist material to remove one ofexposed and unexposed regions of the resist material.
 24. The method ofclaim 23, wherein exposing selected regions of the resist material toenergy comprises exposing selected regions of the resist material to anelectron beam.
 25. The method of claim 20, wherein forming at least oneimage reversal assist feature comprises locating the at least one imagereversal assist feature on the mask at least proximate to a locationcorresponding to a location at a surface of a workpiece to be processedusing the mask at which a phase of an electrical field present at thesurface transitions between phases.
 26. The method of claim 20, whereinforming at least one image reversal assist feature comprises: forming aplurality of image reversal assist features; and centering the pluralityof image reversal assist features about a location on the mask at leastproximate to a location corresponding to a location at a surface of aworkpiece to be processed using the mask at which a phase of anelectrical field present at the surface transitions between phases. 27.The method of claim 20, further comprising forming the at least twoprimary features to have an average width of less than about fourhundred nanometers (400 nm) on the mask, and forming the at least oneimage reversal assist feature to have an average width between about 10%and about 70% of the average width of each of the at least two primaryfeatures.